Monday 17 June 2013

AEO - fundamental challenges

Technology comparison

At the quickfire seminar series at All Energy, tidal energy veteran Peter Fraenkel (Fraenkel-Wright) gave a talk comparing ocean energy technologies. He gave his views on which technologies were the most likely to become commercially competitive. He did not think that OTEC, tidal barrage, or shoreline wave were close to becoming competitive in the near future. Perhaps unsurprisingly, he thought that tidal stream stood a good chance of being competitive with offshore wind. He thought that offshore wave had a chance of becoming competitive, but that it faced greater challenges than tidal stream.


The fundamental challenge of large torque

He then went on to outline the fundamental engineering challenges facing both tidal stream and wave energy: the lower the speed of the prime mover, the higher the torque, and the bigger the machinery. He estimated that the speed of a tidal turbine was an order of magnitude smaller, and the torque per MW an order of magnitude larger, than those typical in fossil fuel plant. He estimated that for wave power, the speeds were again an order of magnitude smaller, and torques per MW again an order of magnitude larger, than those for tidal power.

The connection between torque and speed is a significant issue for tidal power. Peter’s view suggests that tidal power could be made cheaper by designing a turbine with a faster rotational speed. However, he also said that there was an engineering upper limit to blade speed for a tidal turbine, so it is clear that a faster rotor is not the solution. In fact, simply scaling up turbines also results in higher blade tip speeds, and Peter suggested that there was an upper limit for a viable turbine size, which he estimated to be around the 1MW mark. He applauded the Scotrenewables approach of installing several smaller turbines on one large support structure. Joseph Fison (Atlantis), speaking in the Wave&Tidal 7 session of the main conference, also believed that there was a limit to blade size, for what sounded like similar reasons.

The fundamental challenge of the average to extreme ratio

The issue of slow speeds and large torques is not the only fundamental engineering challenge on Peter Fraenkel’s radar. At the recent RenewableUK conference, I asked him why he thought wave energy was lagging behind tidal stream in terms of cost of energy. He answered that the extremes of a resource dictated the project costs, while the average of the resource determined the energy generated (income). There is a smaller gap between averages and extremes of the tidal resource than of the wave resource. Peter believes this to be a key reason why the economics of tidal power is presently better than those of wave power.

Peter Fraenkel’s summation of the fundamental engineering constraints carried the implicit suggestion that wave energy would always be more expensive than tidal energy. It is worth giving this matter some consideration, firstly because it comes from the person who lead MCT to success during its pioneering years, and secondly because tidal energy has indeed been developing faster than wave energy. Next week I will discuss the issues brought up and the questions surrounding these fundamental engineering challenges.

More about All Energy Opportunities 2013:

Image credit:

'Tidal wave ride sign' by Nadia

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